The CECE heavy water production process extracts heavy water from normal water by a combination of electrolysis and catalytic exchange between the water feeding electrolytic cells and the hydrogen produced in them. The CECE process has previously been described in U.S. Pat. No. 3,974,048 issued to Atomic Energy of Canada Limited on Aug. 10, 1976.
The primary components of a normal multi-stage CECE process are each stage's hydrogen water catalytic exchange enrichment columns, oxygen-stream vapour scrubber columns and electrolytic cells. The catalytic exchange columns enrich water flowing down the column by stripping deuterium from the up-flowing hydrogen gas, with conditions always favouring deuterium transfer to the liquid. Electrolytic cells provide a bottom reflux flow by converting the enriched liquid leaving the catalytic exchange column into hydrogen gas. The electrolytic cells in a CECE process not only provide a bottom reflux flow but also enrich the cell liquid inventory.
For economic reasons, the CECE process usually exploits electrolysis installations that are associated with major electrical generating facilities, typically hydro-electric generating facilities. Many electrical power generating facilities are built solely to meet the diurnal peak electricity demand. During off-peak periods, excess electrical generation capacity is diverted to electrolytic cells to produce hydrogen. As the demand for electricity fluctuates through peak and off-peak periods, the availability of electricity for electrolytic processes fluctuates accordingly. This is accommodated by either bringing more electrolytic cells on line or shutting down electrolytic cells as required. In conventional installations where the electrolytic cells are used to produce hydrogen gas, the process can readily accommodate fluctuations in electrolytic cell capacity without adversely affecting the process parameters. However, in installations in which the electrolytic cells are used in a heavy water CECE process, fluctuations in electrolytic cell capacity has a profound effect on the process parameters. In a conventional CECE process, the cascaded separation stages take about 10 hours or more to achieve a steady state concentration profile. By shutting down electrolytic cells to accommodate peak electrical power demand, the concentration profile throughout the CECE stages is disrupted and production of heavy water is substantially reduced.
In CECE installations, a significant portion of the capital cost of the installation is the expense of the catalyst charge in the exchange columns. In a full-scale CECE installation, the catalyst can be 30%-60% of the total investment cost. The economics governing any specific application could dictate that a "turned down" CECE process be used. This is conventionally achieved by using less than the optimum (for full scale) volume of catalytic exchange packing in the first stage columns. As the catalyst volume in the first stage is decreased, the amount of deuterium stripped from the exiting hydrogen gas is reduced resulting in a reduced deuterium recovery.